This map by NASA shows temperature changes over the last 50 years on Antarctica. West Antarctica, separated from the east by a long mountain range, warmed faster. Temperature changes are measured in degrees Celsius.

WASHINGTON — The Obama administration on Monday called for enhanced protection of the Earth's polar regions, proposing mandatory limits on Antarctic tourism and urging increased environmental research there and in the Arctic.

Opening a two-week conference of parties to the 50-year-old Antarctic Treaty, Secretary of State Hillary Rodham Clinton said the collapse Saturday of an Antarctic ice bridge was a stark reminder that the poles are gravely threatened by climate change and human activity.

"With the collapse of an ice bridge that holds in place the Wilkins Ice Shelf, we are reminded that global warming has already had enormous effects on our planet, and we have no time to lose in tackling this crisis," she told the first-ever joint meeting of Antarctic Treaty parties and the Arctic Council at the State Department.

The bridge linking the Wilkins shelf to Antarctica's Charcot and Latady Islands shattered over the weekend after two large chunks of it fell away last year. The shelf, formed by thousands of years of accumulated and compacted snow, had been stable for most of the last century before it began retreating in the 1990s.

Now the size of Connecticut, the shelf on the western side of the Antarctic Peninsula lost 14 percent of its mass last year alone, according to scientists who are looking at whether global warming is the cause of its breakup.

Average temperatures in the Antarctic Peninsula have risen by 3.8 degrees Fahrenheit over the past half century, the statement said — higher than the average global rise.

Clinton said the Antarctic Treaty — which also bars military use of the continent — could be a model for improved cooperation and coordination in the Arctic, which is not governed by a similar pact.

"The treaty is a blueprint for the kind of international cooperation that will be needed more and more to address the challenges of the 21st century," she said.

U.S. wants tighter tourism limitsClinton also formally announced that the United States would be proposing mandatory limits on the size of Antarctic cruise ships and the number of passengers they bring ashore at the treaty conference, which began Monday in Baltimore and runs through April 17.

The move would seek to mandate, under international law, the current voluntary restrictions on tourism.

The State Department says the plan would "minimize the likelihood of marine oil spills" in the Antarctic and "ensure that tourism is conducted in a safe and environmentally responsible manner."

The U.S. proposal contains no specific enforcement mechanism or penalties for limiting tourist operations. But it would require signatories to the pact to ensure that Antarctic tour operators bar ships with more than 500 passengers from landing sites, restrict landings to one vessel at a time per site and limit passengers on shore to 100 at a time.

It would mandate a minimum of one guide for every 20 tourists while ashore, according to the documents.

Limiting tourist access to the continent has taken on urgency because of a surge in visits and recent cruise ship accidents, including two groundings in the just-finished 2008-09 season and the highly publicized sinking of a vessel in November 2007.

Tour operators back limitsThe International Association of Antarctica Tour Operators says visits have risen from 6,700 in the 1992-93 season to 29,500 in the 2006-07 season and 45,213 in 2008-09.

Members of the association first developed the restrictions and adhere to them voluntarily. Members are backing the U.S. proposal for the mandatory limits, which were first adopted by the Antarctic Treaty parties as recommendations in 2007.

"We follow them religiously," said the group's executive director Steve Wellmeier. He acknowledged that without mandated limits, enforcement is "an honor system to a large extent."

The Baltimore meeting marks the 50th anniversary of the pact's signing. Many consider it the first modern international arms control treaty because it says Antarctica cannot be used for military purposes and freezes sovereignty claims on its territory.

The treaty says Antarctica can be used only for peaceful purposes and guarantees freedom for scientific investigations. It sets out guidelines under which the continent can be protected. There are 28 member states and 19 observer countries and organizations to the accord.

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